I'm not entirely sure what you mean by "requires much less refining than gasoline." Diesel is a "mid-barrel" product in the sense that it has a higher average molecular weight and a higher average boiling point than the gasoline fraction. However, that doesn't mean that it takes more effort/energy/time to make gasoline vis-a-vis diesel.

Refining crude oil essentially is separating the components of crude oil into fractions based on boiling point. The lightest fractions boil off at the lowest temperatures (i.e., gases like methane through butane, followed by naphtha). Next is the gasoline fraction (roughly C7 through C11), then the kerosene fraction (roughly C12 through C15), and then diesel, followed by the heavier fuel oils and lubricating oils. Depending on the type of crude oil being refined, about 40% of the crude oil run through the distillation column will be recovered in the gasoline fraction.

Since the heavier components (the so-called "bottoms") aren't worth a lot, the heavier fractions are typically processed further (cracking, hydrocracking) to break apart the bigger carbon chains to smaller carbon chains to make more gasoline and diesel etc. Notably, one would typically not process diesel further to make gasoline. The gasoline fraction may also be further processed or blended to increase the octane number; and similarly the diesel fraction may be further processed to increase the cetane number of the diesel.

As you can see, gasoline does not require "more refining" than diesel.